Molybdenum and its alloys [molybdenum-titanium-zirconium (TZM) and Mo-0.56Nb] were neutron-irradiated in the Japan Research Reactor-2 and the Japan Material Testing Reactor at temperature between 873 and 1263 K to fluences in the range 1.2 to 3.0 × 1024 neutrons/m2 (E > 1 MeV). The effect of these irradiations on mechanical properties was investigated and interpreted in terms of change in microstructure. Radiation hardening caused by voids was observed. The formation of precipitates in the irradiated specimen during the neutron irradiation or during the tension test above 800 K contributed additional hardening. The ductile-to-brittle transition temperature (DBTT) of the irradiated molybdenum and TZM was above room temperature, but below 773 K. The total elongation above 773 K suggested the irradiation temperature dependence of DBTT in the irradiated molybdenum. The DBTT of Mo-0.56Nb alloy was only slightly above room temperature. Annealing this alloy at 1273 K resulted in room temperature ductility to near the preirradiation value. These results suggest that the Mo-0.56Nb alloy is a good candidate material for use as a heat protection wall of a fusion reactor.